In 1998, William Boyd pulled off a famous little hoax. He invented an all-American artist, one Nat Tate, a friend of Jackson Pollock who, in an alcoholic despair, jumped off the Staten Island ferry to his death, most of his potential unfulfilled. Boyd wrote a little book about Tate, full of grainy archive photos apparently of the artist with his cronies, the critic Clement Greenberg and the poet Frank O'Hara. There were paintings, too, a series inspired by Hart Crane's poem 'The Bridge', pictures which Boyd claimed to have discovered the year before.

The joke was carried off with some style: there was an opening at a New York gallery, with critics weighing up the painter's contribution to abstract expressionism; the book came with suitably pretentious quotes from David Bowie and Gore Vidal attesting to Tate's genius; the Sunday Telegraph printed extracts; and, then, after about a week, the game was up, and everyone claimed, not quite convincingly, to have known all the time and to have just been playing along.

In his little monograph on Tate, Boyd suggested that he had been alerted to the artist's work through the writing of 'the British writer and critic Logan Mountstuart, 1906-1991... biographer, belle-lettriste, editor, failed novelist', whose journals he suggested he was editing. The book was full of sly little anecdotes from these journals: 'Bumped into Tate again as I was leaving [the gallery] and complimented him on his work. I asked if he had anything else for sale and he said - most oddly - that I would have to ask his father. Later Pablo shat copiously in the middle of the room, so Larry Rivers told me...' - that kind of thing.

It now turns out, however, that this particular part of Boyd's story at least was 'true': Any Human Heart, a novel, purports to be the compendious collected diaries of the fictional Mountstuart, and comes complete with little introductions by the author, footnotes and an index. It is not clear whether it was conceived originally as an extension of the spoof, or already had a life of its own, but the result is a distinctly odd book: a late-arriving lead balloon to the nicely timed punchline of Nat Tate.

The narrative is made up of half-a-dozen diaries, which are devoted to different periods of Mountstuart's life of ambition and failure: schooldays, war years, dotage and so on. It ranges across the world - the novelist is born in Uruguay, raised in Birmingham and lives subsequently in London, New York, the Bahamas, Switzerland, Africa and the South of France - and takes in the century. It comes from a similar impulse in Boyd as The New Confessions, a novel in which he also tried to gain the form and pressure of our times through one life, though if Rousseau was the loose inspiration there, here it is Montaigne who skulks in the margins.

Mountstuart keeps the Essais by his bedside and his life, as it suggests, appears as simply a parade of shreds and patches of very different lives, dictated by luck and time and place: 'We keep a journal to entrap that collection of selves that forms us,' Boyd, as 'editor', suggests in his introduction, 'the individual human being.'

The problem is that the individual created here never quite becomes vital in any of his lives. Boyd starts brightly and ambitiously enough; the journal of the 17-year-old Mountstuart, at his boarding school, is nicely layered with the pretensions of a particular precocious kind of student - 'Wrote a Spenserian ode on loss of faith. Not very good' - and with adolescent notes to self: 'I shall kiss her this holiday or else become a monk.' Days begin full of hope and poetry and mostly end in masturbation: 'Pleasured myself with delectable visions of Lucy.'

The ambition of this section, though, like Mountstuart's, remains unfulfilled as the life and the journal progresses. Once Boyd is locked into his tyrannical chronology - 'Monday 11 March 1929', and so on - he seems a little mesmerised by it. Thus, occasionally, plodding on through the decades, you have an unnerving sense of the century passing in real time.

Mountstuart has a walk-on part in literary history that becomes all too predictable. He has tea at Garsington with Ottoline Morrell and Virginia Woolf, Evelyn Waugh makes a pass at him, he supplies an unlikely neologism to Joyce in a Parisian bar, shares a beer with Hemingway during the Spanish Civil War, is recruited to the intelligence services by Ian Fleming, and that's all before he fetches up in New York with O'Hara and Pollock and 'Nat Tate'.

Some of this might have been played for laughs, but Boyd seems curiously at pains here to have his readers take Mountstuart as seriously as Mountstuart takes himself. The belle-lettriste is, of course, a friend of Cyril Connolly's, and one model for him might be Logan Pearsall Smith, dedicatee of Enemies of Promise.

Mountstuart is thus seduced from great work, we are led to believe, by nearly all of Connolly's roster of distractions: journalism, early success, domesticity, drink and, mostly, sex. He is married three times, and despairs of his writing as he throws most of his creative energy into a series of ill-fated affairs (in middle age, he typically ejaculates thinking of Balzac's line: 'There goes another novel...').

This feels like wishful thinking. It seems more likely that by far the greater threat to his success as a novelist lies in the stiff-upper-lip prose he displays in his journals. Mountstuart's adventures, notably a run in with Edward and Mrs Simpson, a period in solitary confinement in a Swiss jail and a preposterous interlude with the Baader-Meinhof gang, are all detailed in uniform pipe-and-slippers sentences; very few of his experiences - not murder or incarceration or sexual obsession or extreme poverty - ever really disturb the pleasant equilibrium of this prose. It is as if by inhabiting this other writer, Boyd, lacking his usual easy charm, has sometimes forgotten that he still has to write.

In a way, the author has set himself an impossible task. Even the immediacy and frankness of the century's greatest name-dropping diarists - Kenneth Tynan, Andy Warhol - derive much of their force from the knowledge that the encounters described actually happened. Take that away and you are left with a dull kind of parade. For all the incident, for all the change he witnesses, Mountstuart never really feels like a credible witness either to history or emotion.

Early on in the book, in one of his annual personal statements of account, he makes it his stated goal 'to work, to write, to live'. By the end, you are not really convinced that he has made much of a fist at any of the three.